Protein ubiquitination and deubiquitination have been implicated in the pathogenesis of many human cancers. The effectiveness of the proteasome inhibitor Velcade (bortezomib) in the treatment of multiple myeloma and mantle cell lymphoma establishes the ubiquitin-proteasome system as a valid anti-cancer therapeutic target (Cohen and Tcherpakov, 2010). By using both biochemical and genetic approaches, our lab has demonstrated the roles of HAUSP (a deubiquitinase, also called USP7) in regulating both p53-dependent, and p53-independent tumor suppression. Notably, we have recently developed specific small molecule inhibitors for HAUSP. The proposed studies aim to elucidate the effects and mechanisms of HAUSP inhibitors in both p53-wild type and p53 mutant tumors. In Aim 1, we will examine whether HAUSP is an anti- tumorigenesis target by using HAUSP mutant mouse models. A number studies suggest that HAUSP may be a valuable target in cancer therapy. Nevertheless, this notion has not been demonstrated in any genetic models. In this aim, we will examine whether inactivation of HAUSP inhibits tumorigenesis by activating p53 function in vivo in the classic E--Myc mouse model. In Aim 2, we investigate the role of HAUSP in regulating p53-independent functions in neuroblastoma cells by modulating N-myc stability. In our preliminary studies, we have demonstrated that HAUSP is a bona fide deubiquitinase of N-Myc and that the levels of N-myc protein are tightly controlled by HAUSP. It is well known that N-myc is amplified/or overexpressed in neuroblastomas. To further elucidate the physiological relevance of the HAUSP/N-myc interaction, we will examine the role of HAUSP in regulating N-myc mediated functions in human neuroblastoma cancer cells.In Aim 3, we will examine whether C5, a newly identified HAUSP inhibitor is able to suppress tumor growth in both p53-dependent and p53-independent manners. In our preliminary studies, we have identified a novel HAUSP inhibitor C5 and found that C5 has a much stronger activity (about 100X better than the published leading compound P5901) in HAUSP inhibition. The studies from our lab and others strongly suggest that the targeting of HAUSP activity in human tumors cells should exhibit antitumor efficacy in both p53-dependent and p53-independent manners. We will examine whether this new compound C5 potentially has the abilities to suppress cancer growth in both p53-wt and p53 mutant tumors.